Aqueous base drilling fluid for heaving shale



Patented June .1949

AQUEOUS Bass DRILLING FLUID non HEAVING SHALE Richard A. Salathiel, Houston, Tex, asslgnor, by

mesne assignments, to Standard Oil Development Company, Elizabeth, N. J., a

of Delaware corporation No Drawing. Application January 30, 1948, Serial No. 5,500

4 Claims. (01. 252-85) This invention relates to a drilling fluid which I will effects. minimum of disintegration of heaving shale.

In the rotary drilling of oil and gas wells, fluid is pumped down the drill stem to the drill at the working face in the bottom of the borehole. The stream of drilling fluid passes across the working face of the drill and escapes upwardly through the annular space between the drill stem and the borehole walls. In some areas, as for example 'parts of the Gulf Coast of Texas and Louisiana, formations known as heaving or sloughing shales must be penetrate-7i before the oil or gas reservoirs are reached. I It has frequently been difiicult or impossible to penetrate these formations successfully by the ordinary rotary drilling methods when using common mud fluids. These shales have a tendency to disintegrate by swelling or cracking or both so that the walls of the hole are unstable and material sloughs into the hole or moves into the hole by swelling. The caving or sloughing of material into the hole may frequently result from the unequal swelling and resulting cracking caused by the presence of seams or thin layers of readily hydratable and swellable material closely interbedded with harder and less readily hydratable shale materials. In many cases in the past filling of the borehole, or parts of it, have resulted in stuck drill pipe and abandonment of further efforts to make additional hole. It is common in drillingshale sections which do not swell or slough severely enough to stop drilling operations or even to cause extreme difiiculty to observe extensive enlargements of the borehole or even the formation of subterranean cavities. In these cases although the materials which swell, slough or cave .into the hole are removed by the drilling fluid, the enlarged holes and cavities which are formed interfere to various degrees with operations involved in drilling and completing the well. When such enlargements and cavities are formed the rate f flow of the mud is greatly reduced and the carrying capacity thereof is diminished- To increase the rate of flow, pumps having a large excess capacity must be employed. Furthermore, when it becomes necessary to seal casing, large quantities of cement are required in order to obtain a'pr oper sealing.

It is generally believed that the cause of disintegration of heaving shale is the hydration of shale adjacent the borehole by water contained in the ordinary drilling mud. Thus, a heaving shale may be defined as one which in contact 2 with the usual water base drilling fluids swells or disintegrates continuously to such an extent as to interfere with drilling operations.

The rates of disintegration of sloughin and heaving shales may be decreased when usin conventional drilling muds by adding materials which reduce the rate of filtration loss and improve othermud properties. But with even the best of such conventional drilling muds extreme difl'lculties from shale disintegration are encountered occasionally and serious hole enlargement in shale sections is the rule rather than the exception.

While the disintegration of shales has been overcome to a large extentby the use of certain specially prepared drilling fluids, such for example as sodium silicate base drilling fluids and oil base drilling fluids, the methods heretofore practiced have not been entirely satisfactory. Not the least of the disadvantages of such methodsis the high cost of materials required in their practice. I

It is an object of this invention to Provide a drilling fluid for the drilling of boreholes into subsurface formations by the rotary drilling method through heaving shale without causing the disintegration thereof. 1

In accordance with the present invention the disintegration of heaving shale is prevented by employing a drilling fluid consisting of water, clay, alkali metal hydroxide, salt, and material for promoting the efiectivess of the hydroxide in preventing disintegration of shale. Insteadof adding the alkali metal hydroxide, salt and a material for promoting the effectiveness of the hydroxide to a mixture of water and clay, these materials may be added to ordinary drilling mud provided the drilling mud contains a sufilcient amount of clay to givea fluid having the desired yield point and viscosity after the'above-mentioned constituents have been added. Finely dividedmaterials such as barytes, iron oxide, and the like, may be used to supplement the clay in forming the drilling mud.

A number of different materials may beus ed for promoting'the effectiveness of the alkali metal hydroxide employed and I have fou nd that the alkali metal salts of o'rganic acids having. at least two carbon atoms and not more than fourcarbon atoms arefparticularly suitable. Thus, sodium acetate, sodium. propionate and sodium butyrate are effective for this purpose. The potassium or lithium saltsof the aforementioned acids may also be used.

It is known that when shales are immersed in V suitably strong solutions of alkali metal hydroxides they do not swell. Tests made by placing samples of shales in solutions of sodium hydroxide showed that when substantially less thanthe optimum concentration of sodium hydroxide was used, swelling and sloughing still occurred and little advantage was gained. It was also observed that when substantially more than the optimum concentration of sodium hydroxide was used swelling did not occur but cracking was observed which weakened some of the shales. Shale samples differed among themselves in the optimum concentration of sodium hydroxide solutions required for their preservation. These optimum concentrations for the diil'erent shale samples tested varied within the limits of 15% and 30% by weight of sodium hydroxide. It was further observed that the amount of sodium hydroxide required to preserve a particular shale could be decreased by adding sodium chloride, a much less expensive material, to the solution. Withconcentrations of sodium hydroxide and sodium chloride above the optimum, as with the sodium hydroxide solutions, cracking of some of the shales occurred. It was still further observed that the addition of small amounts of certain salts to the caustic alkali solutions or to the solutions of sodium chloride and caustic alkali, of more than sufilcient concentration to prevent swelling, served to prevent cracking of the shales and thus adapted the solutions to preserving a. wider range of shale types. For example, I have found that a fluid consisting of 70.3 weight percent water, 8.25 weight percent sodium hydroxide, 17.3 weight percent sodium chloride and 4.15 weight percent of an alkali metal salt of the above-mentioned acids is substantially equivalent to a 20 weight percent solution of sodium hydroxide from the standpoint of preventing the swelling of clay and shale samples and superior to such a solution from the standpoint of cracking of some of the shale samples. Thus, the use of such a material as sodium acetate greatly reduces the cracking of shales contacted by alkali hydroxide solutions. It has further been found that the use of sodium acetate makes any given causticbrine solution effective over a wider range of shale types than is the case when no such promoter material is used.

It is to be understood that the amount of the aforementioned alkali metal salts to be employed in the drilling fluid may vary within a, substantial range depending largely on the character of the shale encountered. The minimum concentration of electrolytes which will be effective in any particular case may be determined by experimentation, that is, by obtaining a sample of the shale through which drilling is to be accomplished and observing its rate or degree of disintegration by the action of water containing various concentrations of such salts. When sufficient amounts are incorporated in the drilling mud the rate of disintegration of the shale sample will be negligible. In general, the amount of alkali metal hydroxide may be varied from about 6% to about 15% by weight of the fluid and when such concentrations are utilized the amount of the material for promoting the effectiveness of the alkali metal hydroxide may vary from about 1% to about 10% based on the total fluid, the preferable range of the latter being from about 2% to about 6%. It is to be understood that the concentration of the alkali metal hydroxide required will in general increase as the drier shales are encountered in deep, high temperature formations. Common salt is added to the fluid in quantities suflicient to substantially saturate the fluid after the alkali metal hydroxide and the promoter material have been dissolved therein.

Although any alkali metal hydroxide may be employed, it is preferable to use sodium hydroxide largely because of its commercial availability and cheapness. The common salt referred to above may be either a commercial salt or pure sodium chloride.

While clays do not disperse readily into such fluids, muds of suitable viscosities and gel strengths may readily be prepared by dispersing the clay in water before adding the electrolytes to provide an aqueous phase of the desired composition. When the muds are prepared in this manner, the amount of clay required to give suitable properties is smaller than in fresh water muds. The gelation rate of the muds prepared with these fluids is rapid, and for this reason it is desirable to keep the gel strengths as low as is necessary to hold clay solids and weighting agents in suspension so that cuttings and sand may be settled out and gas cutting of the mud is minimized.

It will be further understood that in the Dractice of my invention other materials may also be added to form the drilling mud. For example, if a heavily weighted mud is required it may be necessary to add weighting materials such as barytes, iron oxide and the like. In addition, treating agents such as sodium hexametaphosphate, tannin, starch, and the like, may also be included in the drilling fluid.

The nature and objects of the present invention having been thus described, what is claimed as new and useful and is desired to be secured by Letters Patent is:

1. A drilling fluid for preventing the hydrous disintegration of shales comprising clay, water, 6 to 15% sodium hydroxide by weight based on the fluid, 1 to 10% by weight based on the fluid of an alkali metal salt of an organic acid having at least two carbon atoms and not more than four carbon atoms for promoting the effectiveness of the sodium hydroxide in preventing disintegration of shale, and sodium chloride in an amount to saturate substantially the said fluid.

2. A composition in accordance with claim 1 in which the alkali metal salt of an organic acid is an alkali metal acetate.

3. A composition in accordance with claim 1 in which the alkali metal salt of an organic acid is an alkali metal propionate.

4. A composition in accordance with claim 1 in which the alkali metal salt of an organic acid is an alkali metal butyrate.

RICHARD A. SALATHIEL.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS 

